1. Field of the Invention
The present invention generally relates to a back-light module for image scanning devices, and in particular to a back-light module with an improved light projecting assembly for projecting a uniform light to the image scanning devices.
2. Description of the Prior Art
Document scanners are generally classified in two types for respectively handling a reflective original document which comprises an opaque substrate and a transmissive original document which comprises a transparent substrate. A transmissive original document scanner comprises a back-light module for generating light projecting the image formed on an original document onto an image sensor system of the document scanner.
A conventional back-light module comprises a movable line-type light source which is movable in a given direction from one end of the original document to an opposite end. A driving system is required to move the light source, which complicates the overall structure of the back-light module.
Another conventional back-light module comprises a surface-type light source which requires no movement of any parts of the back-light module. FIG. 1 is a perspective view that shows an image scanning device 1 equipped with a back-light module 2 comprising a surface-type light source.
As shown in FIG. 1, the conventional scanner 1 for transmissive original documents comprises a housing (not labeled) having a transparent document supporting plate 10 thereon for supporting an original document to be scanned (not shown). A back-light module 3 is pivotally mount ed on the scanner 1 for selectively covering the document supporting plate 10. An optical scanning module 11, comprising a sensor system, is movably supported inside the housing of the scanner by a pair of guide rails 12, 13 for scanning the original document supported on the transparent document supporting plate 10. At scanning of a transparent original document, the back-light module 2 is covered on the document supporting plate 10.
The back-light module 2 includes a casing 21 having an open bottom closed by a bottom plate 22. Within the inner space formed by the casing 21 and the bottom plate 22, there are two tubular lamps, a reflective sheet, a light guide plate, and a diffusion plate. Commonly, the tubular lamps are arranged parallelly along two opposite sides of the light guide plate. Light transmitted from the tubular lamps are guided by the light guide plate for spreading over and projecting from the light guide plate onto the diffusion plate. The light is further distributed by the diffusion plate to uniformly project toward and through the light projecting window 23 to the optical scanning module 11.
The light projecting window 23 mounted on the bottom plate 22 of the back-light module 2 is a light transmissible window. Conventionally, various geometrical patterns of light projecting window are designed according to different requirements, such as rectangular shape light projecting window 23 as shown in FIG. 1 or long rectangular light projecting window 24 as shown in FIG. 2. In scanning operation, a mask with an identical window corresponding to the shape of the original transparent document to be scanned is needed to be placed on the document supporting plate. Therefore, user must prepare a number of masks with different shapes, which is very inconvenient in practice.
Thus, it is desired to provide an improved light projecting assembly for the back-light module for overcoming the above discussed problems.
Accordingly, the primary object of the present invention is to provide an improved light projecting assembly of a back-light module for providing various illumination requirements at scanning different transmissive original documents.
Another object of the present invention is to provide a light projecting assembly for providing a uniform projecting light to the optical scanning module of a scanner. The light projecting assembly is provided with a light projecting window having two symmetric or asymmetric opposite edges.
To achieve the objects above, the back-light module in accordance with a preferred embodiment of the present invention is provided with a light projecting window having two symmetric or asymmetric opposite edges, incorporated with the back-light module. A light guide plate is mounted in the back-light module with a shape corresponding to that of the light projecting window. Two tubular lamps with a shape corresponding to the edges of the light guide plate. Light transmitted from the tubular lamps are guided by the light guide plate for spreading over and projecting from the light guide plate onto a diffusion plate arranged under the light guide plate. The light is further distributed by a diffusion plate and projected to the optical scanning module of the scanner via the light projecting window.
The present invention will be apparent to those skilled in the art by reading the following description of a preferred embodiment and the best mode of operation thereof with reference to the attached drawings, in which: